1. Field of the Invention
The present invention relates to a control circuit for a vibration-type actuator.
2. Description of the Related Art
A vibration-type actuator called a ultrasonic motor or a piezoelectric motor has already been put to practical use. However, in recent years, the vibration-type actuator has been actively developed toward a further improvement in performance. The vibration-type actuator is configured to apply an alternating voltage to an electromechanical energy conversion element such as a piezoelectric element or an electrostrictive element to generate high-frequency vibration in the element and take out its vibration energy as continuous machine motion. The vibration-type actuator is classified into a standing wave type vibration-type actuator and a traveling wave type vibration-type actuator depending on the type of vibration to be generated.
FIG. 1 illustrates a control system for a conventional traveling wave type vibration-type actuator. A command speed is given from a controller (not illustrated), a speed signal representing a relative speed obtained by a speed detector 107 such as an encoder and the command speed are input to a speed deviation detector 101, and a speed deviation signal is output. The speed deviation signal is input to a proportional integral derivative (PID) compensator 102, and a control signal is output. The PID compensator 102 is obtained by adding outputs of compensators respectively having functions of proportion (P), integration (I), and differentiation (D), and is generally used to compensate for a phase delay and a gain of a controlled object and construct a stable and high-precision control system. The control signal output from the PID compensator 102 is input to a driving frequency pulse generator 103 serving as the controlled object. The driving frequency pulse generator 103 generates a driving frequency pulse signal that changes in a driving frequency corresponding to the input control signal. A digital frequency divider circuit, a voltage controlled oscillator (VCO), or the like, is used as the driving frequency pulse generator 103. A pulse signal having a pulse width that changes depending on the control signal may also be generated under pulse width modulation (PWM) control. The driving frequency pulse signal output from the driving frequency pulse generator 103 is input to a driving circuit 104, so that an alternating voltage having two phases that differ by 90 degrees is output. The alternating voltage is an alternating signal having two phases that deviate by 90 degrees. A transformer coupling type voltage booster circuit or a coil/capacitor (LC) boosting circuit having a switching function, for example, is used as the driving circuit 104. The alternating voltage output from the driving circuit 104 is input to an electromechanical energy conversion element in a vibration-type actuator 105, and a moving member in the vibration-type actuator 105 is driven at a constant speed. A driven member 106 connected to the vibration-type actuator 105 is also similarly driven at a constant speed. The speed detector 107 detects a rotational speed. The vibration-type actuator 105 is subjected to feedback control so that the rotational speed comes closer to the command speed.
FIG. 2 is a perspective view illustrating an example of the vibration-type actuator 105. The vibration-type actuator 105 includes a vibrating member 203 composed of a combination of an electromechanical energy conversion element 201 and an elastic member 202, and a moving member 204. Each of the members has an annular shape in a θ direction. When a two-phase alternating voltage is applied to the electromechanical energy conversion element 201, a traveling vibration wave is generated in the vibrating member 203, and the moving member 204 contacting the vibrating member 203 relatively rotates by frictional driving.
FIG. 3 illustrates the elastic member 202. A plurality of protrusions 301 and grooves 302 are alternately provided, as illustrated in FIG. 3, on the side of the elastic member 202 which contacts the moving member 204. In an example illustrated in FIG. 3, 32 protrusions 301 and 32 grooves 302 are provided per circumference. The protrusion 301 is provided so that the amplitude of the traveling vibration wave can be increased at a contact portion with the moving member 204, i.e., at a front end of the protrusion 301. Therefore, a sufficient rotational driving force can be obtained. The protrusion 301 means a relatively convex portion obtained by forming protrusions and grooves that contact the moving member 204.
A feedback control circuit using the above-mentioned PID compensator 102 is widely used for not only the vibration-type actuator 105 but also another controlled object. On the other hand, there is a compensation control method referred to as repetitive control. Japanese Patent Publication No. 06-077201 discusses a repetitive compensator used for a feedback control system to which a target value in a same pattern is repeatedly given for each predetermined period. Registered Patent Publication No. 04152239 discusses a repetitive compensator used for a control system in which a setting value change or a load variation periodically occurs. If the repetitive control is used, an output follows a target input every time the number of repetitive periods increases, so that control with a significantly high absolute accuracy to be required can be obtained. However, there has been no proposal or specific configuration using the repetitive control for the vibration-type actuator. In control of the conventional vibration-type actuator, feedback control is used on the basis of the general PID compensator 102.
However, in the control system using the conventional PID compensator 102, a periodical speed variation corresponding to the number of protrusions 301 of the elastic member 202, which occurs during driving of the vibration-type actuator 105, cannot be sufficiently suppressed.
FIG. 4 is a schematic sectional view of the vibrating member 203 and the moving member 204 during driving. A traveling vibration wave is generated in a rightward direction in the vibrating member 203, and the moving member 204 is driven to rotate in a direction opposite thereto. When a contact portion between the traveling vibration wave and the moving member 204 is enlarged, a front end of the protrusion 301 and a surface of the moving member 204 contact each other, as illustrated in FIG. 4. While contact pressure at the contact portion is ideally always constant during driving, it actually differs due to the irregularities of a plane of protrusion 301 and the moving member 204 depending on a rotational position. More specifically, the contact pressure changes according to the number of protrusions 301 per rotation of the moving member 204. Thus, a variation in a mechanical resonance frequency of the vibrating member 203 corresponding to a contact area distribution between the protrusions 301 and the moving member 204 occurs in synchronization with the rotation of the moving member 204. The variation in the mechanical resonance frequency becomes an amplitude variation of the vibration wave occurring in the vibrating member 203. A periodical speed variation corresponding to the contact area distribution, i.e., the number of protrusions 301 occurs in synchronization with the rotation of the moving member 204.
FIG. 5 illustrates a spatial frequency distribution of a measured speed variation. The horizontal axis indicates a spatial frequency, and indicates the number of fluctuations of a speed variation per rotation (cycles/rotation), i.e., a rotation order. The vertical axis indicates an amplitude (dB) of a speed variation. This result is obtained by performing control using the control circuit for the conventional travelling wave type vibration-type actuator illustrated in FIG. 1, and accepting a speed deviation signal into an external measuring apparatus from the speed deviation detector 101 and analyzing the accepted speed deviation signal. FIG. 5 indicates that peaks occur at positions of spatial frequencies 46 and 92 respectively. The number of protrusions 301 of the elastic member 202 used for measurement is 46. The peak of the spatial frequency 92 is a secondary harmonic component of the spatial frequency 46. The inventors have also made a measurement for a case where the number of protrusions 301 of the elastic member 202 is changed and a case where the rotational speed is changed. In either case, they confirmed that a periodical speed variation corresponding to the number of protrusions 301 occurs.
The vibration-type actuator 105 for driving an electrophotographic drum in a copying machine is picked up here as an example. The moving member 204 is driven at a rotational speed of a maximum of 230 rpm. The vibration-type actuator 105 can perform high-precision speed or position control. If the vibration-type actuator 105 is used as a driving motor for the electrophotographic drum, therefore, an image that hardly shifts in color can be formed. When the vibration-type actuator 105 is driven at the rotational speed of 230 rpm, the speed variation occurs at 176 Hz and 353 Hz when converted into a time frequency. If a speed variation occurs in the electrophotographic drum, color gradation occurs in an image transferred onto paper. This is generally called banding. Gradation occurring in the vicinity of a spatial frequency 1 cycle/mm becomes most noticeable owing to a human luminous characteristic. When the rotational speed of the electrophotographic drum is 230 rpm, a speed variation in the vicinity of 360 Hz becomes the most noticeable frequency as banding. Therefore, not only the speed variation at 176 Hz by the protrusions 301 but also the speed variation at 353 Hz is required to be reduced.
However, the speed variations at 176 Hz and 353 Hz are in a higher band than a control band. Therefore, in a control circuit using the conventional PID compensator 102, a control gain cannot be too much increased, and a speed variation cannot be sufficiently reduced. Generally if the control band exceeds 100 Hz, a phase delay increases, depending on a controlled object including an actuator or a driven member to be used. Therefore, control is difficult to perform at a high gain. Therefore, in the control circuit of the vibration-type actuator 105, a control circuit of separate system for suppressing a periodical speed variation, based on a contact area distribution of the vibrating member 203 is required to be added to prevent banding and implement high-precision speed control.